1. Field of the Invention
This invention relates to an apparatus for continuously purifying crystals by contacting crystalline substance containing impurities with a molten liquid resulted from the crystal component in a counter-current manner.
2. Description of the Prior Art
Various apparatus for purifying crystals have been known and used so far, including those adapted to purify crystals by utilizing a molten liquid resulted from a crystal component, for example, as disclosed in Japanese Patent Publication No. 40621/1972 (British Patent Specification No. 1225711). The apparatus disclosed in the patent publication is a vertical type purifying apparatus comprising means for feeding a substance to be purified including impurities into the lower portion of a tower, in which crystals are melted in the upper portion of the tower, the resulting molten liquid is caused to contact with the crystals upwards in the tower in a counter-current manner and the crystals purified through the contact are taken out from the upper portion of the tower. Two or more rotational shafts are disposed inside the purifying tower, and blade members are disposed spirally around the outer circumference of each of the rotational shafts, so that crystals fed from the lower feed port are transferred upwardly while being disintegrated. A heating portion is disposed, optionally, as means for preventing the deposition of the crystals onto the rotational portion or the wall surface of the tower itself. The apparatus is adapted to purify the crystal substance while rotating the rotary blades in the directions opposed to each other.
When crude crystals containing impurities are purified by using the liquid of the molten aimed crystal component in the crystal purifying apparatus of the foregoing structure, starting crude crystals are fed from the lower portion of the purifying tower and transferred upwardly toward the top of the tower by the rotary blades disposed inside. The crystals reaching the top of the tower are melted by a required amount into a liquid for recycle, which gravitationally moves downwardly in the inside of the tower. Thus, crystals reaching the top of the tower while being washed with the molten liquid are taken out from the top of the tower at a desired purity.
As the result of the analysis and study for the function in the crystal purification, the present inventors have discovered that the crystals are purified by the three types of effects taking place simultaneously, that is, the washing effect in which the surface of crystals rising upwards in the tower are washed through the continuous counter-current contact with the descending molten liquid at high purity, the sweating effect in which impurities contained in the crystals are removed when the crystals are heated and stagnated by the descending molten liquid and the recrystallizing effect in which the descending molten liquid is cooled to recrystallize due to the crystals rising upwards from the lower portion of the tower.
Although it has been found that the washing effect by the molten liquid is extremely remarkable, among the effects as described above, the purity obtained upon purification has a certain limit only with this washing effect. This is shown by the trend of the solid line in the graph of FIG. 1. FIG. 1 is a graph illustrating the relationship between the content of impurities in the crystals obtained by continuously washing crude crystals with a sufficient amount of pure liquid and the quantity of the washing liquid. As apparent from the graph, while the washing effect can be improved by the increase in the quantity of the washing liquid, it can be expected to attain no higher washing effect when the impurity content is decreased to a certain level. While on the other hand, crystals at a purity near the point a in FIG. 1 can be obtained when a certain amount of crude crystals in a eutectic system is heated to a temperature near the melting point and then washed with a pure liquid after maintaining for about one hour. This shows that the sweating effect acts very effectively. Further, in the experiment where crude crystals of a predetermined amount are placed and heated to a predetermined temperature on a metal gauge situated in a container and recrystallized while extracting a portion of a falling liquid, the purity of the thus obtained crystals is extremely high in the same level as above.
Thus, the present inventors have studied on the method of improving the washing, sweating and recrystallizing effects together. However, in the purifying apparatus as described in the Japanese Patent Publication No. 40621/1972, if the functions of these effects are intended to be improved, the purification yield (ratio of the product relative to starting material) is significantly reduced. This may be attributable to the fact that while the foregoing three effects can be improved by increasing the amount of heat in the upper portion of the purifying tower, the molten liquid falls in the tower and is discharged out of the system from the bottom to significantly reduce the yield since this prior apparatus has no cooling means for the crystals.
That is, although the effects of washing, sweating and recrystallization have also been utilized in the prior apparatus, the three effects can not be attained satisfactorily since it has been difficult to crystallize the molten crystal component at the bottom of the purification apparatus. If the molten liquid is cooled only slightly, crystals will be deposited on the cooling surface to hinder the heat exchange.